Preheat oven to 350. Butter a 9 x 5 inch loaf pan and place it on an insulated baking sheet or on two regular baking sheets stacked on top of the other. (This extra insulation will keep the bottom of the bread from over baking.)

Sift together the flour, cocoa, baking powder, salt and baking soda.

Working with a stand mixer, preferably fitted with a paddle attachment, or with a hand mixer in a large bowl, beat the butter at medium speed for about a minute, until softened. Add the sugars and beat for 2 minutes more. Add the eggs one at a time, beating for a minute after each addition. At this point, the batter may look a little curdled -- it's okay. Reduce the mixer speed to low and mix in the mashed bananas. Add the dry ingredients in 3 additions, mixing only until they disappear into the batter. Still on low speed, add the buttermilk, mixing until it is incorporated. Stir in the chopped chocolate. Scrape the batter into the pan.

Bake for 30 minutes. Cover the bread loosely with a foil tent to keep the top from getting too dark, and continue to bake for another 40 to 45 minutes (total baking time is between 70 to 75 minutes) or until a thin knife inserted into the center comes out clean. Transfer the pan to a rack and cool for at least 20 minutes before running a knife around the edges of the bread and unmolding. Invert and cool to room temp right side up.